A steering shaft of a steering apparatus of an automobile is requested to have performances of absorbing axial displacement which is caused during travelling, and not transmitting the displacement and vibrations to a steering wheel. Moreover, such a steering shaft is requested to exert a function of axially moving and adjusting the position of the steering wheel so that the driver can attain a position optimum for driving the automobile. In both the cases, the steering shaft is requested to reduce a rattling sound, a rattling feeling of the steering wheel, and the sliding resistance exerted in an axial sliding operation.
A steering shaft is configured by non-rotatably and slidably fitting a male spline shaft on which male splines are formed, and a female spline shaft on which female splines are formed, with each other. In a male spline shaft, conventionally, axial male spline tooth roots are circumferentially placed on the outer circumference of a columnar shaft member, a resin layer having excellent slidability is molded thereon, and male splines are formed. In the formation of the resin layer for the male splines, the resin layer is molded while the thickness is made thicker, and then a predetermined thickness is attained by a cutting process or the like (Patent Document 1).
In the cutting process in the background art, as shown in FIG. 8, a center pin 28 disposed on a front pressing jig 26, and a center pin 29 disposed on a back pressing jig 27 are firstly fitted into centering tapered holes 30, 31 which are disposed at the shaft centers of the end surfaces of a male spline shaft 21, and the male spline shaft 21 is clamped between the front pressing jig 26 and the back pressing jig 27, thereby supporting the male spline shaft 21.
A cutting tool 25 includes blade portions which are formed in a female spline shape on the inner circumference. The blade portions are disposed on one end side in the axial direction, and directed toward the one end side.
The male spline shaft 21 is forwardly moved together with the pressing jigs 26, 27 with respect to the cutting tool 25, and the resin layer which is formed on the male spline shaft 21 in a thicker manner is scratched off by the cutting tool 25, and finished so as to have a predetermined thickness. After the forward movement, the cutting tool 25 is pulled out from the male spline shaft 21.